In most of the world bilingualism is the norm. Even in the US, a primarily monolingual society, there is a growing awareness that knowledge of a second language is essential to our competitiveness in an increasingly interactive world. However, there are a number of issues concerning the cognitive and neural systems that underlie monolingual and bilingual language use that remain unresolved. This proposal focuses on one specific, but critical component of the mechanisms involved in becoming bilingual - the cognitive and neural processes involved in acquiring and using a vocabulary in a second language (L2). Using both behavioral and electrophysiological (ERPs) techniques, our primary aim is to plot the cognitive and neural consequences of vocabulary acquisition in a foreign language by examining various stages of L2 language learning in both cross-sectional and longitudinal samples of foreign language learners. Reaction time and error data collected in the proposed experiments will allow us to link our data with the large extant literature from prior behavioral studies. The ERP data will help us follow both quantitative and qualitative changes in the processing of L2 words as a function of proficiency. Moreover, by employing this cognitive neuroscience approach it will be possible to more closely tie the cognitive and perceptual processes involved in second language vocabulary acquisition to their underlying neural mechanisms. An important and unique aspect of this proposal is the plan to test two complementary populations of bilingual participants: English native speakers learning French, and French native speakers learning English. This approach will allow unconfounded comparisons of performance in L1 and L2. A major aim of the present project is to test a new model of L2 vocabulary acquisition (the developmental interaction activation model), which predicts three major developmental consequences of second language acquisition in terms of the (re) structuring of form and meaning representations of words in L1 and L2. The three categories of proposed experiments (15 in all) are designed to investigate the developmental trends predicted by this model: (1) unprimed single word recognition experiments manipulating orthographic neighborhood, concreteness, and cognate status of translation equivalents; (2) masked priming studies used to probe the evolution of L2-L1 lexical links and L2 form-concept links; and (3) language switching studies used to probe the evolution of control over the relative activation of words in each language.